The present invention relates to modifying the mineral surface of particulates (including proppants such as sand or synthetic particulates) that are particularly suitable for use in subterranean operations such as gravel packing and fracturing. More particularly, this invention relates to modifying the mineral surfaces of particulates to affect the hydrolysis behavior of hydrolysable coatings thereon.
Particulates of various kinds, and particularly sand, are utilized routinely in carrying out operations in subterranean formations penetrated by well bores. For example, particulates are used for forming gravel packs adjacent well bores, for propping open fractures in a subterranean formation, as structural fillers in cement compositions utilized in well bores and subterranean formations, etc. The formation of a gravel pack adjacent a well bore penetrating a producing formation may cause migrating particulates from produced fluids to be screened out of the fluids thereby preventing plugging of flow passageways, damage to tubular goods and other equipment, etc. In carrying out treatments in subterranean formations whereby fractures are created in the formations to create flow channels therein, solid particulates, referred to as proppant, are commonly placed in the fractures to prevent their closure after fracturing fluid pressure reduction. A variety of other treatments and procedures are carried out in well bores and subterranean formations penetrated thereby using cement compositions, which may include particulates as fillers.
Fairly recently, it has been discovered that such particulates may be coated with hydrolytically degradable materials that may, for instance, produce an acid upon degradation. Such coated particulates may be used in a gravel pack neighboring a filter cake, for example. When the degradable material coating degrades, the acid that is produced may interact with an acid soluble component of the filter cake so as to degrade the filter cake. Another example of a subterranean application wherein such coated particulates are used is breaking (e.g., reducing the viscosity of) a viscosified treatment fluid (e.g., a fracturing fluid or a gravel pack fluid). The acid released by the degradable material upon degradation may assist in breaking the fluid. Examples of such degradable materials include esters, orthoesters, and polyorthoesters. Esters offer the functionality of being delayed release acids; the acids produced can be used downhole for a variety of purposes (e.g., degrading an acid soluble portion of a filter cake or breaking a fluid). Orthoesters and polyorthoesters are believed to be beneficial because they may offer the functionality of a chemical “trigger” for a delayed release acid, meaning that the material can remain inactive for long periods of time, and then become activated at a desired time, oftentimes strongly.
Although using particulates coated with coatings that comprise esters, orthoesters, or polyorthoesters may provide an advantageous means of providing their degradation products to a subterranean formation, it has been found that such particulates may have acidic sites on their mineral surfaces that may catalyze the hydrolysis of the coatings, causing them to hydrolyze prematurely. The acidic sites are thought to be due, at least in part, to the presence of dangling —OH groups on the surface of the particulates that form hydroxyl groups in water; these hydroxyl groups can be acidic. In some instances, these dangling —OH groups may form silic acid with water. Premature hydrolysis of the esters, orthoesters and polyorthoesters is undesirable, inter alia, because it can undermine the purpose for which these coatings are being used. For example, if these are used in a fluid loss control pill, the pill may degrade prematurely, and therefore, not prevent fluid loss as desired. Additionally, premature hydrolysis can undermine the desirable chemical trigger aspect of orthoesters and polyorthoesters. To achieve a delayed hydrolysis, it may be necessary to affect the acidic sites on the particulates so that they do not catalyze the hydrolysis of the coatings on the particulates, for example, by neutralizing them or by making them slightly basic.